Explained most simply, tides are the result of the gravitational pull of the moon on the earth. Since liquids change shape much more easily than solids, and because the moon is constantly rotating around the earth, the oceans periodically swell and thin corresponding to the moon's direction.

A big source of confusion is why tides also appear on the opposite side of the earth than the moon. To explain this, we'll have to get into a bit more specifics. Much like with magnets, gravity weakens the further you get from an object. The result of this is that the gravitational force exerted by the moon is weaker on one side of the earth than the other. Let's imagine that the moon stopped rotating about the earth, and the two planets began to fall toward each other. Looking from the center of the earth, the side closest to the moon would experience a stronger acceleration, and would be pulled away from the earth slightly (causing a tidal bulge). The side furthest from the moon would experience a weaker acceleration, and would 'lag behind' the rest of the earth when falling towards the moon. Viewed from the center of the earth, this side would also look like a tidal bulge. The constant 'falling' of the earth's orbit thus results in moving tides on both sides.

With this knowledge, we can also explain why the sun has a weaker tidal effect than the moon. While the gravitational effect of the sun on the earth is much, much greater, since the sun is much further away, the difference in force between one side of the earth and the other is actually smaller, 45% smaller, in fact. The additional effect of the sun on tides results in changes to the magnitude of tides. Spring tides, with the sun and moon in closer alignment, are typically much higher.

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Tides occur because relative to the CENTER of Earth, the point on Earth’s surface beneath moon feels a bigger tug due to Moon; this is because the point on the surface of Earth is CLOSER to the center of the moon, than the center of the Earth is.

Similarly, the point OPPOSITE to the moon on Earth surface feels a tug SMALLER than that relative to the center of Earth; so, the point directly under the moon is pulled towards moon, and the point on opposite side is pulled away from the moon.

Gravity between the earth and moon creates a slight "bulge" in the oceans on the side of the earth facing the moon. Since the moon is orbiting the earth (about once a day), the location of the bulge moves, too, and you get different tide levels throughout the day. I hope that helps!

Throughout the day, the oceans are constantly moving, rising and falling. This change in the level of the water is called the tide. And the tide is controlled not only by the moon, but also by the sun, the way the earth rotates, and more. The most important factors affecting tides are gravity and rotation, and as the moon rotates around the earth because of gravity, the moon is said to “control” the tides.

But to understand how the moon’s gravity and rotation cause water to rise and fall as tides we need to understand a bit about water. Water molecules (every individual molecule that makes up the ocean and every body of water) are attracted to each other in a way that makes them stick together (think about dew droplets on a plant leaf in the morning; the water forms this droplet because the different molecules of water stick together). This property of water is called “cohesion”, and because of this property of water, all water molecules want to stick together and then they act like a single body. So rather than every water molecule in the ocean acting in different ways, all of the molecules stick together and act together.

As the earth rotates, the force of the rotation pulls the ocean water away from the earth’s surface (you can test this by holding your arms out and spinning as quickly as you can. Do your arms feel like they want to fly off of your body?) Now we know that the ocean water molecules all stick together which means that every molecule in the ocean pulls away from the earth’s surface. This creates a mound of water around the center of the earth (the earth’s equator) and a thinner layer of water at the arctic and the Antarctic poles. If the earth was all alone in it’s rotation then we would just have a mound of water at the middle of the earth and less water at the north and south pole; however, because the moon is very close to the earth, its gravity has an effect on this bulge. So when you watch the water level rise in Santa Barbara (as the tide rises) it means that the rotation of the earth is carrying California away from the moon because the beach that we are watching is being rotated toward the part of the earth where the ocean has a water mound. As we move right in to the bulge of water we have a high tide. Then as the earth rotates and carries Santa Barbara away from that water mound, the tide falls and we end up in a low tide. So why do we have two high and two low tides every day? because there are actually two mound of water on opposite sides of the earth. It takes the earth 24 hours to complete one full rotation and in those 24 hours we pass through these two mound of water (2 high tides) and two low points in the water (2 low tides). And that mound of water is caused by the gravitation force of the moon on the earth as the spot where you are on the earth rotates past the moon.

The force of gravity increases the closer you are to the object exerting the gravitational force. The side of the Earth closer to the moon is therefore being pulled on more strongly than the side of the Earth facing away from the moon. Oceans, being made of water, are free to move when pulled on by gravity, which causes them to pile up on the side of the Earth facing the moon, which causes tides.

Oceans also pile up on the side facing away from the moon. The pull is weaker over there, and intermediate in the parts of the Earth where the moon is on the horizon. The water where the moon is on the horizon has been pulled to the side facing the moon, causing a low tide there. Thus, there are two high tides and two low tides.